1. Field of the Invention
The invention relates to a device for reading flexible storage foils.
2. Background Art
Recently, flexible storage foils are being used instead of X-ray films. When ionizing radiation or X-rays impinge on such foils, metastable storage centers will be produced, which are lattice defects or color centers (or generally trap centers), which have trapped a charge carrier (electron or hole) produced by the ionizing radiation. Such storage centers are stable over long times. If the storage centers are illuminated with a very narrow laser beam of corresponding wave length, the storage centers will be moved into a higher excited state, from which the charge carriers can recombine under emission of light called photo-stimulated luminescence (PSL). The latter process is also shortly referred to as recombination of storage centers.
At such points of the storage foil, whereon a larger amount of X-rays has impinged, one obtains by reading this point using a reading light beam, a higher amount of light quanta than at such points, which have received only a few X-rays. If the storage foil is scanned in two dimensions, the output signals of a light detector receiving the PSL corresponds to the optical density of a conventional X-film.
In known reading devices, two dimensional scanning of the storage foil is obtained by arranging the storage foil on the outer surface of a drum, by rotating the drum and by moving a reading unit along a generating line of the drum. The reading unit includes a laser source and a light detector.
Such drum type scanners, which are also known for scanning images, are disadvantageous in that they have larger moving masses and in that the scanning velocity which can be achieved is only small due to this fact so that the scanning process takes a long time. It is thus an object of the present invention provide a reading device, wherein the moving masses are smaller and which allows high scanning velocities and short scanning times.
In a reading device in accordance with the present invention the storage foil support has the form of a part cylinder or of a cylinder, and a light deflecting element is arranged on the axis of this cylinder""s surface. This deflecting element produces a fine rotating reading light beam, which scans the interior surface of the storage foil. This light deflecting element requires only very small dimensions and is of small mass, only. Due to this construction, the reading device in accordance with the present invention can work well with higher speed or rpm.
These and other objects of the present invention will become apparent in light of the present specification, claims and drawings.
If in accordance with the present invention, a pentaprism is used as the deflecting element for the reading light beam, one obtains a particularly precise deflection. The reflection of the reading light beam is exactly at 90xc2x0 with respect to the axial direction of irradiation and into a radial measuring direction not withstanding whether the prism is exactly aligned or not. Also play of a bearing journaling a prism carrying shaft has no influence on the deflection of the reading light beam. Thus motors of simple construction showing some play of the shaft can be used for rotating the light deflecting element without impairing the precision of the deflection of the light.
The improvement of the present invention allows the use of the light deflecting element also for focusing the reading light beam onto the interior surface of the storage foil. In one preferred embodiment, there is a reading light source which already per se provides a reading light beam of very small cross section and small divergence. This makes it possible to construct the light deflecting element as a very small component.
A further improvement of the invention is advantageous in view of a compact structure of the reading device and makes it possible to arrange the reading light source also at a distance from the axis of the cylindrical surface.
In a preferred embodiment of the invention, both mirrors of the reading device, which deflect the light, which is provided by the laser on an axis being parallel to the axis of the support surface, exactly onto the axis of the support surface, are in fixed relative position since the two deflecting mirrors are part of a single rigid optical element. This is advantageous in view of reducing the adjusting steps.
A further improvement is advantageous in that only a single deflecting mirror is necessary to provide an incoming laser beam on the axis of the foil support.
In another preferred embodiment, the reading light beam of a semiconductor laser diode has a circular cross section which results in pixels of the scanned image which have equal dimension in the two scanning directions.
The present invention is advantageous in view of utilizing as much of the fluorescence light as possible. A further advantage resides in the fact that the efficiency in detecting fluorescence light, which is emitted along the scanning circle by the storage foil, is constant. Thus no subsequent corrections of the detected fluorescence signals are necessary.
The light detector of the present invention may have a smaller radius so that the costs for the detector are smaller. In spite of this advantage, the light generated at a larger radius can still be used due to the annular mirror reflecting this light onto an opposing mirror which will then reflect the light into the light detector.
The improvement of the present invention is also useful in view of using as much of the fluorescence light as possible, which light is emitted by the storage foil after illumination with laser light.
In a preferred embodiment of the invention, the light deflecting element can be driven directly without an intermediate direction changing gear by a motor that is arranged behind or in the mirror opposing the light detector.
In this respect, the improvement of the invention is advantageous in that such light is guided to the light detector, which impinges onto the mirror under large angles (grazing impingement). Thus the detection efficiency for fluorescence light is increased. Since the measured intensity of the fluorescence light is proportional to the intensity of the laser light as well as proportional to the detection efficiency, one can reduce the intensity of the laser light and still obtain the same sensitivity of demeasuring system. This is advantageous in that low cost laser light sources can be used.
In accordance with the present invention, the mirror opposing the light detector can also serve as an absorbing layer for reading light. Thus, undesired reflections of the laser light can be avoided, which could result in storage centers lying in regions of the foil, which are not yet to be scanned, being already caused to fluoresce. This would result in poorer resolution of the image. Also the contrast of the storage foil would be noticeably impaired.
The geometry of the mirror can have a large radial extension without having large axial extension and without requiring thin wall sections in the radial outward portion thereof. Also, transport means provided to feed the storage foils across a reading gap defined by the foil support, can be arranged close to the axial end of the mirror which is advantageous in view of precisely advancing the storage foil in axial direction at the location of the reading gap.
In such a preferred embodiment, reflected light will not travel in circumferential direction for a longer time but will be diffusely reflected to the light detector.
If the mirror is a cast component, the optical surfaces of the mirror can be already provided in the casting process. These surfaces need none or very little final treatment.
The present invention is likewise advantageous in still further reducing the amount of reading light which reaches the light detector.
In another preferred embodiment of the invention, as much of the fluorescence light as possible is detected by increasing the overall detecting surface. Thus a maximum amount of fluorescence is made available for the production of electric signals.
It is contemplated that solutions for driving the light deflecting element in a way that the light deflecting element and the drive motor associated thereto require only little space.
In a preferred embodiment of the invention, PSL originating from the scanning circle (intersetion of the plane of rotation of the reading light beam and the light sensitive interior surface of storage foil bent to cylindrical or part cylindrical geometry) is used for generating an electric signal in both half spaces, i.e. on both sides of the plane of rotation of the reading light beam.
In the present invention, reading light is kept way from the detector. In addition, undesired reflections of reading light are avoided, which could read the storage foil at other points distant from the actually scanned point and which could thus result in faulty reading of the storage foil.
The storage foil to be read out is arranged on the outer surface of the foil support member. In spite of this fact, the reading light beam has complete access to the interior surface of the storage foil throughout 360xc2x0.
In a preferred embodiment of the invention, very simple arrangement of the storage foil on the foil support and the force generated in the elastically bendable storage foil warranting a snug contact of the storage foil on the support surface of the foil support is allowed. This is advantageous in view of reducing imperfect definition or sharpness of the image which may result from unprecise radial positioning of the storage foil outside of the focusing circle of the reading light.
Furthermore, better protection of the light detector against ambient light is provided for.
If the light blocking brush element is formed, movement of the storage foil through the light barrier formed by the brush element is possible under small friction and thus small wear.
In another preferred embodiment, advantageous solutions for keeping the storage foil in safe surface contact with the supporting surface of the foil carrier without mechanically affecting the front side of the storage foil which is prone to formation of scratches is contemplated.
The present invention likewise provides a solution as to providing the axial movement of the storage foil with respect to that transverse plane wherein the reading light beam rotates in a simple manner.
The improvement of the present invention is useful in that the danger of tilting of the storage foil under the influence of the transport means acting in axial direction is eliminated.
The improvement of the invention is advantageous in view of a good and reliable frictional contact between the transport means and the storage foil.
In a preferred embodiment, a large area of contact between the transport means and the storage foil results, such that uncontrolled slip between the transport means and the storage foil is avoided.
A reading device of the present invention is useful in that no reading light can escape. Furthermore no ambient light can reach the light detector without being attenuated.
The improvement of the present invention warrants that reading light, which possibly transverses the storage foil (in the case of a storage foil having no absorbing back layer) or which otherwise reaches the shielding member or a foil guiding member, will be absorbed and will not be reflected back to the storage foil, which might again result in faulty reading as has been pointed out above. The indicated construction of the shielding member and/or the foil guiding member allows to use also such storage foils which do not include a back layer absorbing the reading light.
The present device allows feeding of small storage foils, e.g. storage foils replacing conventional dental intraoral X-ray films directly to the working run of the axial drive means without exerting special diligence.
In the present device, positioning of the small storage foils is made at a point close to the input end of the axial drive means so no misalignment may occur on the way between the positioning means and the input end of the second drive means.
The present device allows tactile positioning of the small storage foils.
Furthermore, there is a smooth transition between the positioning means and the support surface of the foil support.
In a preferred embodiment of the invention, a plurality of small storage foils can be read out simultaneously.
Measuring the angular position of the read out light beam using a mechanical or optomechanical position encoder would mean a very costly encoder considering the desired resolution of the image. Also this encoder would have to measure the angular position at a high measuring rate. It is contemplated that a reliable and precise signal being representative for the angular position of the reading light beam is obtained using simple mechanical and electronic components.
In a device of the present invention, at least regions of the image(s) are rapidly stored in a memory. This allows preprocessing and rejection of signals which do not correspond to pixels of the desired image reading the device already before forwarding the image signals to a computer for further processing.
In a device of the present invention, the actual dark current of the light detector is measured continuously. In accordance with the measured dark current, a dark current threshold value can be set which is used in setting the image signals associated to image points receiving no light to zero.
In a device of the present invention, the flux of data to be communicated to an external processor is reduced. This allows use of a relatively slow interface which is commercially available. Averaging of successive image signals is also advantageous in view of improving the signal to noise ratio.
In a preferred embodiment of the invention, the number of image signals combined into an averaged signal can be varied. Normally high resolution of the image is required in connection with dental intraoral images which are of small size, while a somewhat reduced resolution is acceptable in connection with dental panoramic images. So the total amount of information to be handled by the electronics associated to the device is about the same in connection with panoramic images and intraoral images.
In such a device the extent of averaging is established automatically in accordance with the size of the foil to be scanned.
Recognition of the foil size is particularly simple in accordance with the present invention. If a small storage foil is recognized in the foil positioning means of the foil support the device is set into the high resolution mode.
A further improvement of the invention is also advantageous in view of reducing the flux of data communicated to an external processor.
In a device in accordance with the present invention, the image signals associated to a plurality of small size intraoral storage foils can be rapidly stored in an image signal memory of the device itself. Transfer of the image data to an external data processing unit can then be made at a smaller rate using a commercial interface.
In a preferred embodiment of the present invention, only those of the output signals provided by the light detector are used, which correspond to image points of the storage foils, while those portions of the output signals which correspond to positions of the reading point which are outside of the storage foils arranged on the foil support are discarded. Recognition of the edges of the storage foils can be simply achieved by detecting a succession of a given number of non zero image signals by the data reduction circuit.
In accordance with storage foils of different size or different nature it is desirable to vary the gain of the light detector. This can be achieved using a device in accordance with the present invention.
In a device of the present invention, the gain of the light detector is automatically adjusted responsive to the size of the storage foil arranged on the foil support, the size being an indicator for the sensitivity of the storage foil and for the dose conditions during exposure.
In such a device the detector gain can be wholly or at least partially adjusted manually. This allows some basic adjustment of the detector gain in accordance with local scanning conditions and in accordance with the type of storage foils and optical densities a particular dentist or doctor prefers to use.
The present invention is advantageous in view of the little space used by the device. Also there is some gravity feeding of the storage foils in regions, where there are no positive axial drive means. It is also advantageous in view of ease of removal of read out storage foils. Furthermore, the read storage foils are particularly easy to grasp.